This project continues to involve the biochemical characterization of protein tyrosine kinases (PTK) cloned from natural killer cells. The primary emphasis of this project is the study of a PTK that has significant homology to the carboxyl-terminal Src kinase (Csk); the Csk homologous kinase, Chk (previously known as Lsk). Before the discovery of Chk, Csk was the only PTK known to phosphorylate the conserved carboxyl-terminal tyrosine of Src family kinases and down-regulate their catalytic activity. Unlike Csk, which is ubiquitously expressed, Chk is expressed primarily in hematopoietic cells. We have shown Chk expression to be inducible in both T cells and peripheral blood monocytes. In T cells our studies have shown that whereas Csk is known to inhibit the signal transduction of the T cell receptor, Chk expression does not inhibit T cell receptor function. In an effort to elucidate the biochemical basis of the differences between Csk and Chk in the T cell compartment, we have compared the abilities of the Csk and Chk Src homology 2 domains to bind tyrosine phosphorylated proteins from human blastic T cells. These studies have demonstrated that the two kinases bind overlapping but distinct patterns of phosphoproteins. Further studies have identified one of the Chk binding proteins as paxillin. immunoprecipitation and fusion studies confirmed that the Chk-paxillin interaction is likely direct and requires protein tyrosine phosphorylation. Consistent with these findings expression of Chk in Csk -/- fibroblasts reduces the tyrosine phosphorylation of paxillin. In addition to T cells, we have been investigating the role of Chk in monocytes/macrophages using Chk -/- mice. Our studies have demonstrated that macrophages from Chk -/- mice respond to interleukin(IL)-1 stimulation with increased production of IL-6 relative to wild-type mice. Consistent with these findings, preliminary data suggest that challenge of Chk -/- mice with turpentine, a model of sterile inflammation, results in increased levels of serum IL-6 and enhanced production of the acute phase protein serum amyloid A. Further in vivo and in vitro studies are under way to elucidate the mechanisms behind the functions of Chk in the myeloid cells. Together our studies should continue to shed light on the biochemical and molecular control of the immune response.